I would guess that the zip you uploaded is not correct. I just extract the viewer.xml from the zip file and then replace it in the original so I'm not re-zipping anything. If you do have to unzip and the rezip the entire package, then you need to make sure that you aren't zipping up a folder with the presentation in it. The index.html and other files need to be in the top level of the zip. Basically if you open your new zip and see a folder, it is not done correctly. If you open your new zip and see something similar to this (see below) it was done correctly.